Characterization of rickettsial infection in Amblyomma americanum (Acari: Ixodidae) by quantitative real-time polymerase chain reaction

Several species of the spotted fever group Rickettsia (SFGR), with considerable variation in vertebrate host pathogenicity, are present in ticks in the United States. In this study, quantitative real-time PCR (qPCR) was used to characterize the growth and the distribution of Rickettsia amblyommii in selected tissues (salivary glands, gut, and ovaries) of naturally infected Amblyomma americanum (L.) (Acari: Ixodidae), during bloodmeal acquisition and throughout vertical transmission to eggs and postembryonic life cycle stages (larvae and nymphs). R. amblyommii was identified in the samples at ratios of < or = 1 rickettsiae per tick cell. Significant variability in the ratio of rickettsial to tick gene copy numbers between the tissues was identified; however, no single tissue was consistently observed to have the greatest rickettsial burden throughout the feeding event. Furthermore, the ratio of rickettsial to tick gene copy numbers did not significantly differ between eggs, immature ticks, and feeding events. This is the first study to use qPCR to enumerate rickettsial growth and distribution in the tick host during bloodmeal acquisition. Deciphering SFGR tissue distribution and transmission mechanisms is necessary for the development of novel approaches to control tick-borne rickettsial diseases.